A lithium ion secondary battery is lighter in weight and has higher capacity than a nickel-cadmium battery, a nickel hydrogen battery, or the like. For this reason, the lithium ion secondary batteries have been widely used as a power supply for mobile electronic appliances. The lithium ion secondary battery is also a strong candidate for a power supply to be mounted on hybrid automobiles and electric automobiles. Moreover, with the size reduction and higher sophistication of the mobile electronic appliances in recent years, the lithium ion secondary battery used for the power supply is expected to have higher capacity.
The capacity of a lithium ion secondary battery mainly depends on an active material of an electrode. As a negative electrode active material, graphite is typically used. To meet the above demand, however, it is necessary to use a negative electrode active material with higher capacity. In view of this, metal silicon (Si) with much higher theoretical capacity (4210 mAh/g) than that of graphite (372 mAh/g) has attracted attention.
An example of the negative electrode active material using such metal silicon includes a mixture of silicon and silicon oxide. The stress due to the expansion and contraction of the silicon in the mixture of silicon and silicon oxide at the time of charging and discharging is relieved by silicon oxide. Thus, this mixture is supposed to be superior to silicon in cycle characteristics. In the mixture of silicon and silicon oxide, an irreversible reaction involving reduction of an oxide occurs at the time of initial charging, and therefore the amount of electricity required for the reduction constitutes an irreversible capacity. As a result of such irreversible reaction, the lithium that contributes to discharge is decreased, and therefore the capacity corresponding to the amount of decrease in lithium constitutes an irreversible capacity. The amount of lithium contributing to charging and discharging is primarily determined by the amount of lithium in the positive electrode. Accordingly, an increase in the irreversible capacity in the negative electrode leads to a further decrease in the capacity of the battery as a whole.
In order to decrease the irreversible capacity, a technique has been proposed whereby, prior to the start of charging and discharging, metal lithium is brought into contact with the negative electrode in advance so as to dope the negative electrode with lithium (lithium pre-dope) (see Patent Literatures 1 and 2, for example). Patent Literature 1 discloses a method for doping the negative electrode with lithium by forming a lithium-containing film on the negative electrode. Patent Literature 2 discloses a method for doping the negative electrode with lithium by having lithium particles contained in a negative electrode active material layer.